Navy Lifer then wrote to me offline and it adds to the further discussion so I'm posting it here:
<BLOCKQUOTE>quote:</font><HR> I've checked--93 9C1 (master cylinder diam) is 1.125", which I believe lends further support to myscenario. The only connection to the 94-96 cars is that the 93 car used the D614 pad, also on 94-96 Impala (RPO JB9). <HR></BLOCKQUOTE>
Then I'd say your idea, needing more MC swept volume to feed the longer throw 9C1 front discs is the correct ones.
Actually, I had written
<BLOCKQUOTE>quote:</font><HR> The rear [9C1] discs require more fluid volume to operate the caliper pistons to full travel than rear drums, so you need the 9C1 master cylinder.<HR></BLOCKQUOTE>
and that's still true. In addition to that, the front 9C1 calipers also demand more fluid because of the longer throw pads (as you reminded me), so I'll say it was for BOTH reasons.
To go any further with this, we'd have to compare the fluid demands for the disc/drum Caprice front calipers and rear drum wheel cylinders, and then compare the swept total volume at the wheels (from min wear to max wear) and as theroetically provided by the MC, against the WX3 with disc/disc and against the 9C1 with disc/disc.
That would be a neat exercise is sanity checking the work of the GM brake engineer(s).
Now if YOU want to look these up, by all means go ahead. <vbg> - Ken